Sources

Sources of air pollution

We understand air pollution, above all, as a dynamic process with a range of actors and factors – from the purely physical to human action to belief systems. No single factor on its own is likely to lead to air pollution but rather, it is a combination of a wide range of very different factors acting together that come together in the phenomenon we call air pollution. 

Our strategy should be to make an inventory of all contributing factors and actors and factors and determine where we might effectively intervene to get the best possible result for the lowest potential cost and effort.

At a physical level we might cite the rugged topography of the Sóller valley, a bowl-like depression surrounded by high peaks. 

And, meteorologically, we have the phenomenon of thermal inversion. 

Thermal inversion. 

Generally, atmospheric temperature decreases with height. But sometimes, the opposite happens: the bottom of a valley registers colder temperatures than the summits of the mountains. 

During the day the sun warms the air near the earth’s surface. Normally this heated air expands and rises during the day, diluting low-lying pollutants and carrying them higher into the atmosphere. Air from surrounding high-pressure areas then moves down into the low-pressure area created when the hot air rises. This continual mixing of the air helps keep pollutants from reaching dangerous levels near the ground.

Sometimes, however, a layer of dense, cool air is trapped beneath a layer of less dense, warm air in a valley. This is thermal inversion. In effect, a warm-air lid covers the region and prevents pollutants from escaping in upward-flowing air currents. 

Temperature inversions are frequently a result of other weather conditions in an area. They occur most often when a warm, less dense air mass moves over a dense, cold air mass. This can happen, for example, when the air near the ground rapidly loses its heat on a clear night. The ground becomes cooled quickly while the air above it retains the heat the ground was holding during the day. 

Topography can also play a role in creating a temperature inversion since it can sometimes cause cold air to flow from mountain peaks down into valleys. This cold air then pushes under the warmer air rising from the valley, creating the inversion.

Contaminants

But what of the contaminants themselves? These are often the result of human activity – though not always: the arrival of air from the Sahara frequently bings high levels of PM 10.

 Vehicles and fossil fuels

Cars, trucks, and buses powered by fossil fuels are major contributors to air pollution. Fine particles — less than one-tenth the diameter of a human hair — pose a serious threat to human health, as they can penetrate deep into the lungs. PM can be a primary pollutant or a secondary pollutant from hydrocarbons, nitrogen oxides, and sulfur dioxides. Diesel exhaust is a major contributor to PM pollution. But car and trucks also emit volatile organic compounds (VOCs) that eventually get converted into particulate matter through chemical reactions in the atmosphere. 

Non-exhaust emissions (NEE) are particles released into the air from brake wear, tyre wear, road surface wear and resuspension of road dust during on-road vehicle usage. Tight regulation of exhaust emissions by the EU has meant that while cars are now emitting less particle pollution, tyre wear pollution remains unregulated and can be 1,000 times worse. Harmful particle matter from tyres–and also brakes –is a very serious and growing environmental problem, one that is being exacerbated by the increasing popularity of large, heavy vehicles such as SUVs, and growing demand for electric vehicles, which are heavier than standard cars because of their batteries. The stop-start nature of town traffic makes things worse.

 Burning of gricultural waste

Agricultural management is another human activity that contributes particulate matter to the atmosphere. Of course, people living off the land, all those farming it, ensure that the rural world remains a living, dynamic place. Farmers provide local produce and create and maintain the landscape. It is crucial that their work continue being a productive sector. But like every productive sector, it also produces waste – cuttings, branches and so on. And fire is often the tool used to dispose of such waste. Other alternatives such as collection may be hindered by physical factors such as inaccessibility of terrain.

Wood-burning stoves

Finally, wood-burning stoves can make a significant contribution to air pollution. One study found that a single wood burner generates twice the number of harmful PM2.5 particles than a diesel exhaust. A paper by the British Medical Journal found that just one “wood log burning stove” sends more PM2.5 into the atmosphere than a “1,000 petrol cars. But emissions vary widely depending on how the design of the stove, stove is lit and the humidity of the wood.

 Other factors

Other factors are clearly budgetary: projects to collect farm waste rather than burn it place demands on the public purse, while programs aimed at replacing old wood-burning stoves through grants may be costly.

Finally, a range of sociological and anthropological factors may contribute to the process. There may be a belief that old ways are natural and good – that they can’t be unhealthy. Fire is also seen in many cultures as having cleansing and purification properties. Wood-burning stoves and the smell of wood smoke is often nostalgically viewed and is associated with cosy evenings.

Any efforts to address the process whereby the air is polluted must take into account all these factors. Some, such as physical geography, are clearly beyond our control. Others are subject to intervention. Collectively, we need to decide where we might best intervene for the greatest good.